1. Field of the Invention
The present invention relates generally to a radio receiver with a phase-locked loop (PLL) for a demodulator and, more particularly, to a radio receiver with a phase-locked loop demodulator in which a desired signal is easily tuned even in the presence of an adjacent undesired signal with high electric field intensity during a tuning operation.
2. The Prior Art
There has been proposed in the art a radio receiver with a phase-locked loop as a demodulator. However, generally speaking, since a voltage-controlled oscillator (VCO) used in the phase-locked loop is changed in accordance with fluctuations of ambient temperature and source voltage, in order to perform stable reception of signals, the lock range, which is the frequency range at which the phase lock of the phase-locked loop is released by a shift of the frequency of an input signal after the phase-locked loop has been phase locked to an input signal, must be relatively wide. However, there is a proportional relationship between the lock range and the capture range, which is the frequency range at which the voltage controlled oscillator of the phase-locked loop is captured by the frequency of the input signal from its free-running frequency, so that if the lock range is wide, the capture range is also wide.
Generally speaking, if an undesired radio-frequency signal with high electric field intensity is too close in frequency to a desired signal that is supposed to be selected by a radio receiver, it is difficult to detect the desired signal. In a radio receiver in which a phase-locking loop is used as a demodulator, the phase-locking loop may be locked to the undesired, or jamming signal that has a high electric field intensity, thereby preventing reception of the desired signal by the receiver. This often occurs in the case of a phase-locked loop with a wide capture range.
Further, it is noted that in a radio receiver that uses a phase-locked loop as a demodulator, the voltage controlled oscillator of the phase-locked loop is subjected to a modulation operation by a beat component between a desired signal and a jamming signal. As a result, the modulated carrier or component is demodulated by the demodulator and adversely affects the signal-to-noise (S/N) ratio of the audio output signal. This is generally referred to as two-signal jamming characteristics. For example, if the desired selected signal having an angular frequency of .OMEGA..sub.c from the intermediate frequency amplifier circuit of a radio receiver contains a crosstalk signal with an angular frequency of .OMEGA..sub.x, the output signal from the phase comparator circuit of the phase-locked loop will include beat components with angular frequencies of .OMEGA..sub.c + .OMEGA..sub.x and .OMEGA..sub.c - .OMEGA..sub.x. In this case, the beat component with the angular frequency of .OMEGA..sub.c + .OMEGA..sub.x is eliminated by a low-pass filter in the phase-locked loop, but the beat component with the angular frequency of .OMEGA..sub. c - .OMEGA..sub.x is not eliminated by the low-pass filter and is applied to the voltage controlled oscillator as a control signal. As a result, the oscillation frequency of the voltage controlled oscillator is subjected to frequency-modulation operation by the beat component of .OMEGA..sub.c - .OMEGA..sub.x. Thus, the voltage controlled oscillator may produce signal components with angular frequencies of .OMEGA..sub.c + (.OMEGA..sub.c - .OMEGA..sub.x), .OMEGA..sub.c - (.OMEGA..sub.c - .OMEGA..sub.x) and so on in addition to the necessary oscillation signal with an angular frequency of .OMEGA..sub.c. These signal components are applied to the synchronous detector circuit of an AM demodulator and hence the crosstalk signal is demodulated or the demodulated signal from the crosstalk signal is contained in a demodulated signal as a whole to deteriorate the two-signal jamming characteristics.
To reduce such a defect, it is sufficient to narrow the pass band of the IF amplifier, but this causes the high frequency component of the demodulated signal to be attenuated, which deteriorates the reproduced sound.